1. Field of the Invention
The present invention relates to power converters generally and more specifically to a rectifying network for use therein, the network having improved reverse recovery characteristics.
2. Description of the Prior Art
Semiconductor diodes have less than ideal characteristics. Consider, for example, their use in prior-art-type power converters (processors), particularly the topologically similar converters known as boost, buck and buck-boost. (Not only are the above-mentioned converters topologically similar but E. E. Landsman in an article entitled "A Unifying Derivation of Switching DC-DC Converter Topologies" published in the proceedings of the 1979 Power Electronics Specialist Conference, San Diego, Calif., of June 1979 on pages 19-21 discloses how the converters may all be represented by a simple canonical switching cell.)
Boost converters are commonly used to develop, from a voltage source, an increased-level potential for use by a load. Typically, boost converters employ an energy storing inductor connected between a node (terminal) for connection to a voltage source and a central node; a diode connected between the central node and a node (terminal) for connection to a load; and a transistor connected as a switch between the central node and a node common to the source and load (circuit ground). A pair of filter capacitors are also employed, one being connected from the source node to the common node and the other being connected from the load node to the common node. Additionally, a circuit is employed for driving the transistor-switch so as to maintain the potential developed at the load node at the increased level.
Buck converters, which are commonly used to develop a reduced-level potential, typically employ the same components in rearranged form. Specifically, the transistor-switch is connected between the source and the central nodes. Additionally, the diode (or a load-line-correcting network) is connected between the central and common nodes; and, the inductor is connected between the central and load node.
Finally, should the inductor, which may be thought of as a one-to-one auto transformer, of the boost converter by replaced by the primary winding of a transformer having a secondary winding across which the diode and the (load filter) capacitor are series connected, the converter is of the buck-boost configuration.
The above-mentioned prior-art-type boost converter is illustrated in FIG. 1 of the drawing generally designated by the number 10. Shown with the basic components are their associated parasitic elements. As previously indicated, the basic components include the energy storing inductor, which is designated 20 and which is shown connected between the source node (terminal), designated 22, and the central node, designated 24; the diode, which is designated 26 and which is shown connected to the load node (terminal), designated 28; the transistor-switch, which is designated 30 and which is shown connected between node 24 and circuit ground (the common node); the source and load filter capacitors, respectively designated 32 and 34; and the transistor driving circuit which is designated 36 and which is shown connected to (the gate of the transistor of) transistor-switch 30, to node 28, and to circuit ground.
The parasitic elements include the inductor 20 (parasitic) shunt capacitance, represented by a capacitor 50, the inductor 20 (parasitic leakage inductance, represented by an inductor 52 connected in series with capacitor 50 across inductor 20; the transistor-switch 30 equivalent shunt capacitance represented by a capacitor 54 connected in parallel with transistor-switch 30; the (parasitic) wiring inductance (associated with the path from inductor 20 and transistor-switch 30 to capacitor 34), represented by an inductor 56 connected in series with diode 26 between nodes 24 and 28; the diode 26 shunt capacitance, represented by a capacitor 58 connected in parallel with diode 26; the capacitor 32 (parasitic) inductance, represented by an inductor 60 connected in series with capacitor 32 between node 22 and circuit ground; and the capacitor 34 (parasitic) inductance, represented by an inductor 62 connected in series with capacitor 34 between node 28 and circuit ground.
The capacitance of (parasitic) capacitor 50 is normally several orders of magnitude less than the capacitance of capacitors 32 and 34. The (parasitic) inductance of inductor 52 may, or may not, be significant as compared to that of (parasitic) inductors 60, 56 and 62. The capacitance of each of capacitors 54 and 58 increases approximately exponentially as the voltage developed across the respective capacitor decreases. Finally, the inductance of each of (parasitic) inductors 60 and 62 is normally several orders of magnitude less than that of inductor 20.
Shown in FIG. 2 generally designated by the number 100 is a waveform illustrating the rise in the central node (node 24) potential following the time driver 36 initiates "turn-off" of transistor-switch 30, a time designated 102. At any instant, the rate of rise of the node potential is determined by the instantaneous current flow through inductor 20 and the sum of the instantaneous capacitances of capacitors 50, 54 and 58. As illustrated at 104, the node 24 potential rises slowly at first because the capacitance of capacitor 54 is relatively large. However, as the node 24 potential increases, the capacitor 54 capacitance decreases, increasing the rate at which the potential rises, as illustrated at 106. If transistor-switch 30 is configured around a device of the MOSFET type and if the current flow through inductor 20 is of the order of several amperes to several tens of amperes (or more), the rate of rise of the node 24 potential can exceed 10 volts per nanosecond. If the inductance of each of (parasitic) inductors 52, 56, 60 and 62 is negligible (small), the node 24 potential overshoots slightly, as illustrated at 108, the equilibrium potential, illustrated at 110, the equilibrium potential exceeding the node 28 potential (load potential) by the diode 26 forward conduction voltage drop. If the inductance of (parasitic) inductor 56 and/or 62 is significant, the overshoot is greater, as the current flow through (parasitic) inductor 60 develops a voltage drop across (parasitic) inductor 56 and/or 62 for a period of time sufficient to cause the current flow through inductors 56 and 62 to reach the steady state value of the current flow through inductor 20 (E=L(dI/dT)). The so-called forward recovery time of diode 26 may add to the overshoot. Depending upon the particular diode, the contribution may be negligible or may be in the order of volts to tens of volts (or more) for nanoseconds to microseconds.
By a judicious choice of components, the inductance of (parasitic) inductors 60 and 62 may be made small enough to be negligible; however, the layout and the construction of diode 26 (what steel is present in the package and where) dictate the values of the inductance of (parasitic) inductor 56 and the capacitance of capacitor 58. Obviously, at least during the transistor-switch 30 "turn-off" period, it is important that the inductance of (parasitic) inductor 56 be made as small as possible, ideally approaching zero.
A waveform shown in FIG. 3 generally designated by the number 200 illustrates the fall of the central node (node 24) potential following the time driver 36 initiates "turn-on" of transistor-switch 30, a time designated 202. During a first period, designated 204, the node 24 potential drops, as indicated at 206, as dictated by the magnitude of the inductance and resistance associated with transistor-switch 30, diode 26, capacitor 24 and their inter connections. During this first period (period 204) transistor-switch 30 draws not only the current flowing through inductor 20 but also a reverse current flowing through diode 26.
Thereafter, during a second period, designated 210, the node 24 potential tends to "hold", as indicated at 212, at an intermediate potential "V.sub.R " which is related to the reverse dynamic impedance of diode 26. It is during this second period (210) that the charge is "swept" out of diode 26. During this second period (period 210), the power dissipation in the transistor-switch 30 transistor is high as the transistor is conducting a large current (oftentimes several times the current flowing through inductor 20) while supporting a voltage drop, V.sub.R.
Finally, after the charge has been "swept" out of diode 26 and the diode assumes a high reverse impedance state, during a third period, designated 220, the node 24 potential falls at a relatively fast rate, as illustrated at 222, governed by the current transistor-switch 30 draws from capacitors 50, 54 and 58 (in a fashion analogous to the "turn-off" of the transistor-switch). If transistor-switch 30 is configured around a device of the MOSFET type, the rate of fall of the node 24 potential may exceed 10 volts per nanosecond.
An inductor interposed between node 24 and diode 26 would tend to limit the peak reverse current conducted through diode 26 as well as support a voltage drop during the second period (period 210). As a result, the power dissipated by the transistor of transistor-switch 30 would be reduced (as well as that dissipated by diode 26) increasing the overall converter 10 efficiency. Unfortunately, to be effective, the required interposed inductor inductance would have to be tens or hundreds of times larger than the maximum inductance usually tolerable for good circuit operation during transistor-switch 30 "turn-off" periods. For a further discussion of the use of an interposed inductor with a complex circuit employing secondary windings and diodes to couple current pulses to storage elements, the reader is referred to the article by L. G. Meares which was published in the Ninth Power Conversion Conference paper B-2.
Some semiconductor diodes tend to oscillate when subjected to a high reverse dV/dT just after completing their reverse recovery. The oscillation may persist for many tens of cycles often at a frequency at, or near, the self-resonant frequency or capacitor 34 (shown in FIG. 1) and (capacitor 34 parasitic) inductor 62. When the inductance of (parasitic) inductor 56 is very small, when the inductance of (parasitic) inductor 60 and the capacitance of capacitor 34 is small, and when the capacitance of capacitor 50 and/or 54 is significant (large), the amplitude of the oscillation may be comparable to, or exceed, the level of the voltage source (connected to node 22), high enough to cause component failures. The oscillation mechanism is believed to involve a space charge trapped within diode 26 when the depletion region rapidly formed during the third period (designated 220 in FIG. 3), possibly being similar in nature to IMPATT or TRAPATT oscillations.
The oscillation is difficult to suppress using conventional R-C damping methods. However, an inductor interposed between node 24 and diode 26 can be effective in controlling the oscillation, particularly when the interposed inductance is much larger than the inductance of (parasitic) inductor 62 and/or where the interposed inductor inductance is sufficiently large (to act as a low-pass filter with the relatively large capacitance of capacitor 58 at low reverse bias so) that diode 26 is not subjected to a high instantaneous dV/dT during the third period (period 220).
Semiconductor diodes of the type which are designated 15R4 by Solid State Devices Inc. and those which are designated UES 703 by Unitrode Inc. were observed under similar operating conditions, specifically, a forward current of from 5 to 20 amps and a peak reverse current of 10 to 30 amps (during the second period, period 210). Diodes of the 15R4 type exhibited a reverse recovery time of about 8 nanoseconds; and, diodes of the UES 703 type exhibited a reverse recovery time of approximately 20 nanoseconds (being not quite as "crisp"). Both types of diodes exhibited oscillations in the 50-80 Megahertz region. The use of an interposed inductor (between node 24 and diode 26) to reduce the dV/dT during the third period (period 222) from approximately 20 volts per nanosecond to approximately 5 volts per nanosecond was effective in suppressing the oscillations. An interposed inductor inductance of about 10-20 nanohenrys reduced the oscillations significantly; and, an inductance of about 1 microhenry eliminated the oscillations. Unfortunately. as previously indicated, an interposed inductor inductance of even a hundred nanohenrys is usually intolerable for good circuit operation during transistor-switch 30 "turn-off" periods.